The increased threat of a bioterrorist attack in recent years highlights the critical need for the development of potent vaccine formulations to protect the susceptible population. Vaccine formulations contain antigens that induce immunity against pathogenic agents. However, immune responses to many antigens, while detectable, are frequently of insufficient magnitude to afford protection against a disease process mediated by the agents expressing those antigens. In such situations, it is necessary to include an adjuvant along with the antigen in the vaccine formulation.
An adjuvant is a compound that, when used in combination with specific vaccine antigens, potentiates the resultant immune response. The mechanism of action of adjuvants is not precisely known, and may not be the same for all adjuvants. However, it is believed that adjuvants prolong the bioavailability of an antigen. Adjuvants also seem to increase the size of the antigen, thus increasing the likelihood of phagocytosis. Additionally, most adjuvants have a stimulatory effect on the cell-mediated branch of the immune system, i.e., on T lymphocytes (T cells).
There are two well-defined subpopulations of T cells: T cytotoxic (Tc) cells and T helper (Th) cells. T cytotoxic cells kill intracellular pathogens. On the other hand, Th cells exert most of their functions through secreted cytokines. T helper cells are further divided into Th1 and Th2 cell types. Differences in cytokine-secretion patterns of the Th cell types determine the type of immune response made to a particular antigen challenge.
In general, Th1 cells stimulate cytotoxic responses against intracellular viruses, bacteria and protozoa via secretion of interferon-gamma (IFN-γ) and other pro-inflammatory cytokines. The cytotoxic responses include the activation of Tc cells. In contrast, Th2 cells are induced by allergens and helminth parasites, and are characterized by the secretion of interleukins, e.g., IL-4, IL-5, etc. Both Th cell types stimulate the humoral branch of the immune system, i.e., the B lymphocytes.
Different pathogens elicit different types of cell-mediated immune responses. For example, infecting mice with a helminth parasite polarizes the immune response to Th2 activation. In some cases, the polarization is so potent that a Th1-dominant response to an infectious pathogen can be inhibited by the introduction of a helminth parasite. (Brady et al “Fasciola hepatica suppresses a protective Th1 response against Bordetella pertussis” Infect. Immun. 67: 5372-5378 (1999).) Similarly, a Th1-mediated mouse autoimmune disease can be ablated by introducing a helminth parasite into mice (Cooke et al. “Infection with Schistosoma mansoni prevents insulin dependent diabetes mellitus in non-obese diabetic mice” Parasite Immunol. 21:169-176 (1999)).
Additionally, the anti-inflammatory properties of the products of two helninth parasites have been shown to be capable of down-modulating inflammatory Th1responses in mice. In particular, body fluid from the pig roundworm parasite, Ascaris suum, potently stimulates cytokines characteristic of Th2 cells. (Paterson et al., “Modulation of a Heterologous Immune Response by the Products of Ascaris suum” Infect. Immuunol. 70:6058-67 (2002)). Also, a secreted glycoprotein product, ES-62, of a rodent parasite has been found to have broad anti-inflammatory properties that inhibit Th1 cytokine production in experimentally-induced arthritis in mice (McInnes et al., “A Novel Therapeutic Approach Targeting Articular Inflammation Using the Filarial Nematode-Derived Phosphorylcholine-Containing Glycoprotein ES-62” J. Immuunol. 171:2127-33 (2003)). This product is currently being developed as a novel anti-inflammatory therapeutic.
Recently, two helminth products have been reported as acting as adjuvants. Both are strong inducers of Th2 responses to bystander proteins in a vaccine. In particular, proteins secreted by adult Nippostrongylus brasiliensis (a parasite of rodents) were found to be strong inducers of Th2 responses in mice immunized with an unrelated protein (Holland et al., “Proteins secreted by the parasitic nematode Nippostrongylus brasiliensis act as adjuvants for Th2 respones” Eur. J. Immunol. 30 (7):1977-1987 (2000)). Similarly, lacto-N-fucopentaose III, a carbohydrate found on the surface of the eggs of a human parasite, Schistosoma mansoni, acted as a Th2 adjuvant for a bystander protein when injected into mice (Okano et al., “Lacto-N-fucopentaose III Found on Schistosoma mansoni Egg Antigens Functions as Adjuvant for Proteins by Inducing Th2-Type Response” J. Immunol. 167:442-450 (2001)).
Until the present invention, products from helminths have been found to be potently Th2 dominant. Accordingly, their use as adjuvants has been to induce the Th2 cell type responses. Although Th2 cell type activation is important, Th1 cell type activation is critical for the efficacy of certain vaccines. In addition to providing a different cytokine profile than that provided by Th2 cells, Th1 cells activate cytotoxic effector mechanisms which Th2 cells do not activate.
Moreover, other adjuvants presently used in human vaccines also are not effective in stimulating cytotoxic responses to intracellular pathogens. These adjuvants include aluminum salts, e.g., aluminum potassium sulfate, aluminum phosphate and aluminum hydroxide. Without the ability to stimulate cytotoxic responses to intracellular pathogens, the use of such adjuvants is limited.
In additional to protecting against infectious diseases, vaccination is becoming significant in other developing technologies. These technologies include, for example, vaccination against syngeneic tumors. In such new approaches, it is important to be able to induce different types of immune responses.
Accordingly, there is a critical need for safe and effective adjuvants and therapeutics capable of boosting immune responses to a wide variety of pathogens and against tumors. There is a particular need for adjuvants that boost Th1 cell type responses.